Depending on the force balance of the actuator unit and the return spring the needle opens or closes a nozzle and in that way controls the injection of fuel. In an increasing number of applications actuator units with a piezoelectric actuator are used. They have the advantage of having a very fast response time to actuating signals and like that enable multiple injections into a cylinder of the internal combustion engine during one working cycle of the cylinder.
Piezoelectric actuators are very sensitive to fuel. For that reason they need to be sealed from the flow of fuel or other fluids. In this respect it is known to join one of the free ends of a metal bellow to the needle, for example by welding, and to join the metal bellow on its other free end to another element and in that way to establish a sealing. However materials for the needle that are well-suited for welding are often not well-suited for a reliable long-lasting operation of the needle in view of frequent opening and closing of the nozzle.
WO 03/016707 A1 discloses a fluid injector with a connector to a fuel supply, a housing, an actuator unit, and a valve body. The housing is double tubed and has a recess, which takes in the actuator unit. The actuator unit comprises a piezoelectric actuator, which acts on the needle. Between the walls of the double tube-shaped housing the fuel is led from the connector to a fuel inlet of the valve body. The valve body has a housing part with a recess, that takes in a needle. Depending on the position of the needle a nozzle is opened or closed and respectively fuel is injected or not.